The present invention generally applies to signal processing, and particularly applies to processing received signals using Fast Walsh Transforms.
Newer wireless communication standards, such as the evolving Wideband Code Division Multiple Access (W-CDMA) standards, incorporate increasingly sophisticated signal encoding and modulation schemes in efforts to increase bandwidth efficiency and transmission data rates. These advances enable users of the newer wireless networks (e.g., third-generation) to receive and transmit relatively high-rate data streams representing a variety of information types, including voice, data, and video.
Efficient spectral use requires these newer wireless networks to adopt variable data rates where, in essence, users are assigned the minimum data rates necessary to support their respective communication needs. For example, a user receiving a multimedia presentation from the Web is allocated more bandwidth and/or a more complex signal-encoding scheme than a user engaged in a simple voice call.
Various approaches, such as those adopted by the W-CDMA standards, employ orthogonal variable spreading factor (OVSF) codes to achieve variable data rate transmissions. In such schemes, one of a set of orthogonal spreading factor codes is used to spread user data. Such spreading factors might range from 4 to 256, for example. In W-CDMA, the chip rate is 3.84 Mchips/s, and the data rate (pre-spreading) equals the chip rate divided by the chosen spreading factor (SF). Therefore, user data rates are varied by appropriate selection of spreading factors.
Additional data rate variability derives from the use of multiple code assignments per user, referred to as multi-code spreading. For example, in W-CDMA a user might be assigned multiple channels to increase the aggregate data rate to that user. One approach to such multi-code spreading involves the use of different channelization codes per assigned channel (e.g., different OVSF codes per channel). In this manner, multiple dedicated physical data and/or control channels (DPDCH, DPCCH) can be assigned to each user to increase the maximum data rate to that user.
While such capability is good from the perspective of enhanced performance, receiver complexity necessarily increases with the variable spreading factors and multi-code transmissions. Indeed, the use of multi-code transmissions requires wireless communication receivers in such systems to accommodate multi-code received signals that require essentially separate despreading operations for each multi-code component of the received signal. Such variability in the received signal places demands on the receiver in terms of adapting its signal processing to the changing signal parameters.